Conventional mechanical transmissions, particularly those for heavy-duty trucks, typically use a gear shift lever coupled to the transmission via a mechanical linkage to effect a ratio change or shifting of the transmission. In these transmissions it is desirable to minimize the effort required by the driver to shift the transmission to reduce driver fatigue while still maintaining an acceptable "feel" for the driver. More recently, automated and semi-automated mechanical transmissions have been developed which utilize one or more remote actuators to effect a gear change in the transmissions. In these transmissions, it is desirable to reduce shift force so that the associated actuators may be reduced in size and/or power required, resulting in cost and weight reduction of the overall vehicle system.
Various transmission shifting mechanisms have been developed and applied to both the traditional (manual) mechanical transmissions and the automated or semi-automated mechanical transmissions. Shifting mechanisms which utilize a single shifting shaft or rail for selecting and engaging the ratios of a multiple speed vehicular mechanical transmission are known in the prior art as may be seen by reference to U. S. Pat. Nos. 4,621,537; 4,532,823; and 4,222,281, the disclosures of which are hereby incorporated by reference. Such mechanisms may have advantages relative to multiple shift rail devices such as those disclosed in U.S. Pat. Nos. 4,445,393; 4,275,612; 4,584,895 and 4,722,237, the disclosures of which are hereby incorporated by reference. These advantages may include reduced complexity (fewer parts), the ability to use improved bearings and better surface finishes since only a single shaft is involved, a smaller package, and relatively easier control of an auxiliary transmission including sensing of transmission conditions such as neutral and gear-state conditions.
Regardless of the particular shifting mechanism used, gear change transmissions generally use a shift bar housing assembly. Typically, such shift bar housings are manually controlled and operated by a shift finger fixed to a directly mounted shift lever or to the cross shaft of a remotely controlled shifting mechanism. To prevent selection of more than one gear at any particular time, interlock mechanisms are provided. The shift bar housing assembly contains one or more axially moveable shift bars, shift rails, or shift rods. Each shift rail carries at least one shift fork, which is axially moved to engage or disengage a selected transmission gear. The shift rail (or shift fork in a single shift rail transmission) is generally selected by a transverse (left or right) movement of an actuator or shift lever. The selected gear is engaged or disengaged with an axial or longitudinal (forward or backward) movement of the shift rail which causes an axial movement of a clutch member, a gear carrying clutch teeth, or a jaw clutch, as well known in the prior art, and may be seen by reference to U.S. Pat. Nos. 4,445,393; 4,754,665; 4,876,924; and 5,053,961, the disclosures of which are hereby incorporated by reference.
Shift bar housing assemblies which are not directly operated by the driver are also known in the prior art. Such assemblies may be actuated by pressurized hydraulic fluid, pressurized air, or electric motors and have suitable controls therefor, as may be seen by reference to U.S. Pat. Nos. 4,428,248; 4,445,393; 4,722,237 and 4,873,881, all assigned to the assignee of the present invention and all hereby incorporated by reference.